In the manufacture of integrated circuits (IC), or chips, patterns representing different layers of the chip are created on a series of reusable photomasks (also referred to herein as masks) in order to transfer the design of each chip layer onto a semiconductor substrate during the manufacturing process. These layers are built up using a sequence of processes and translate into the tiny transistors and electrical circuits that comprise each completed chip. Thus, any defects in the mask may be transferred to the chip, potentially adversely affecting performance. Defects that are severe enough may render the mask completely useless. Typically, a set of 15 to 30 masks is used to construct a chip and can be used repeatedly.
A mask employed in optical lithography generally comprises a transparent substrate having an opaque, light-absorbing layer disposed thereon. Conventional masks typically include a glass or quartz substrate having a layer of chromium on one side. The chromium layer is covered with an anti-reflective coating and a resist. During a patterning process, the circuit design is written onto the mask, for example, by exposing portions of the resist to an electron beam or ultraviolet light, thereby making the exposed portions soluble in a developing solution, if the resist is a positive-tone resist. The soluble portion of the resist is then removed, allowing the underlying chromium and anti-reflective layers to be etched (i.e., removed).
With the shrinking of critical dimensions (CD), present optical lithography is approaching a technological limit at the 28-nanometer (28-nm) technology node (or N28). Next generation lithography (NGL) is expected to replace the current optical lithography method, for example, in the 20-nm technology node (N20) and beyond. There are several NGL candidates. Of these NGL candidates, multiple electron beam direct writing (MEBDW), which is also referred to as multiple electron beam maskless lithography (MEBML2), and extreme ultraviolet lithography (EUVL) are leading candidates. Although it is claimed that MEBDW does not require a mask, MEBDW requires delineating 1× feature sizes directly on the substrates, which could be quite challenging for advanced technology nodes with very fine features. EUVL uses a much shorter wavelength, such as about 13.5 nm, which is about 1/10 the effective wavelength of ArF immersion lithography, to enhance resolution. The EUV exposure tools (EUV scanners) also utilize reduction projection printing as optical scanners and may achieve a reduction ratio of 4. As a result, EUVL is considered as a most promising candidate for NGL.